Spinach (Spinacia oleracea L.) is a flowering plant from the Amaranthaceae family that is grown as a vegetable. The consumable parts of spinach are the leaves and petioles from the vegetative stage. Spinach is sold loose, bunched, in pre-packed bags, canned, or frozen. There are three basic types of spinach: industry-, fresh and Asiatic spinach. Within these types three different leaf types can be recognized: namely the savoy, semi-savoy and smooth types. Savoy has dark green, crinkly and curly leaves. Flat or smooth leaf spinach has broad, smooth leaves. Semi-savoy is a variety with slightly crinkled leaves. The main market for spinach is baby-leaf. Baby spinach leaves are often of the flat-leaf variety and usually the harvested leaves are not longer than about eight centimeter. These tender, sweet leaves are sold loose rather than in bunches. They are often used in salads, but can also be lightly cooked.
Downy mildew—in spinach caused by the oomycete fungus Peronospora farinosa f. sp. spinaciae (formerly known as P. effusa)—is a major threat for spinach growers, because it affects the harvested plant parts, namely the leaves. Infection makes the leaves unsuitable for sale and consumption, as it manifests itself phenotypically with yellow lesions on the older leaves and a greyish fungal growth on the abaxial leaf. The infection can spread very rapidly and can occur both in glasshouse and in soil cultivation. The optimal temperature for formation and germination of P. farinosa f. sp. spinaciae spores is 9 to 12° C., and it is facilitated by a high relative humidity. When spores are deposited on a humid leaf surface they can readily germinate and infect the leaf. Fungal growth is optimal between 8 and 20° C. and a relative humidity of >80%, and within 6 and 13 days after infection mycelium growth can be observed. Oospores of P. farinosa can survive in the soil for up to 3 years, or as mycelium in seeds or living plants.
In recent years various resistance genes or R-genes have been identified that provide spinach plants with a resistance against downy mildew, as described in e.g. US2013230635, WO2015036378, WO2015036469, and EP2848114. Co-evolution of plant and pathogen has led to an arms race in which a R-gene mediated resistance can be broken down as a consequence of the capability of the pathogen to interact with and modify alternative host targets or the same targets in a different way. In any case, the recognition is lost and infection can be established successfully resulting in disease. In order to re-establish resistance in a plant, a new R-gene has to be introduced which is able to recognize the mode of action of an alternative pathogenicity factor.
This shows that the durability of such R-genes is relatively low, especially since the last few years the development of new races of spinach downy mildew has increased rapidly.
To date 16 pathogenic races of spinach downy mildew (Pfs) have been officially identified and characterized, and many new candidates are observed in the field. The 16 officially recognised races of Peronospora farinosa f. sp. spinaciae, are designated Pfs:1 to Pfs:16 (Irish et al. Phtypathol. Vol. 98 pg. 894-900, 2008; Plantum NL (Dutch association for breeding, tissue culture, production and trade of seed and young plants) press release, “Benoeming van Pfs:14, een nieuwe fysio van valse meeldauw in spinazie”, Sep. 19, 2012; Report Jim Correl (Univ. Arkansas) and Steven Koike (UC Cooperative Extension, Monterey County), “Race Pfs:14—Another new race of the spinach downy mildew pathogen”, Sep. 18, 2012; Plantum NL press release, “Denomination of Pfs:15, a new race of downy mildew in spinach”, Sep. 2, 2014, Plantum NL press release, “Denomination of Pfs:16, a new race of downy mildew in spinach, Mar. 15, 2016). Races 4 to 15 were identified between 1990 and 2014, while only recently another new Peronospora isolate has been identified, termed UA201519B, which subsequently has been officially named Pfs:16 by the International Working Group on Peronospora (IWGP) (Plantum NL (Dutch association for breeding, tissue culture, production and trade of seed and young plants) press release, “Denomination of Pfs:16, a new race of downy mildew in spinach”, Mar. 15, 2016. All 16 officially recognized Pfs races are publicly available from the Department of Plant Pathology, University of Arkansas, Fayetteville, Ariz. 72701, USA, and also from NAK Tuinbouw, Sotaweg 22, 2371 GD Roelofarendsveen, the Netherlands.
Given the fact that R-gene mediated resistance provides a resistance of relatively low durability and the fact that the downy mildew pathogen is evolving and adapting more rapidly to these R-genes, there is a need in the art to have resistance sources available which are more durable compared to R-gene mediated resistance.
Next to an R-mediated defense mechanism, a plant can exhibit a more basal form of resistance which may prevent the pathogen from infecting the plant. This non-R-gene mediated form of resistance can be considered as an extremely successful form of defense which in fact is effective for most plant pathogen interactions. Since such a resistance is not based on the kind of interaction and recognition between host and pathogen as is known for R-genes, this defense mechanism provides resistance against a broader spectrum of pathogen races than would normally be expected with R-genes.
Citation or identification of any document in this application is not an admission that such document is available as prior art to the present invention.